1. Field of the Invention
The present invention relates to a magnetic recording medium having a high recording capacity, a high access speed and a high transmission speed. In particular, the present invention relates to a magnetic recording medium for data-backup, which records and reads data with a reading head comprising a magnetoresistance element (hereinafter referred to as xe2x80x9cMR headxe2x80x9d).
2. Prior Art
Magnetic tapes have various applications such as audio tapes, videotapes, computer tapes, etc. In particular, in the field of magnetic tapes for data-backup, a tape having a memory capacity of several ten GB or more per one reel is commercialized with the increase of the capacity of hard discs to be back-upped. Therefore, it is inevitable to increase the capacity of the tapes for data-backup. It is also necessary to increase the traveling speed of a tape and a relative speed between the tape and a head to increase an access speed and a transfer speed.
To increase the capacity of a tape for data-backup per one reel, it is necessary to increase a recording density in a width direction with decreasing a track width (a signal pattern width on the tape) to 15 xcexcm or less in addition to the prolongation of a tape length per one reel with decreasing the thickness of the tape and the reduction of thickness demagnetization and thus the decrease of a recording wavelength with reducing the thickness of a magnetic layer to 0.3 xcexcm or less.
When the thickness of the magnetic layer is reduced to 0.3 xcexcm or less, the durability of the magnetic recording media tend to deteriorate. Therefore, at least one primer layer is provided between a non-magnetic support and the magnetic layer. When the recording wavelength is shortened, the influence of spacing loss between the magnetic layer and the magnetic head increases. Thus, if the magnetic layer has large projections, a half width value of output peak (hereinafter referred to as xe2x80x9cPW50xe2x80x9d) may increase or the output may decrease due to the spacing loss so that an error rate may increase.
Since a leaking magnetic flux from the magnetic recording medium decreases when the recording density in the width direction is increased with decreasing the track width to 15 xcexcm or less, a MR head, which can achieve a high output even at a minute magnetic flux, is used as a reading head.
Examples of the magnetic recording media used with the MR head are disclosed in JP-A-11-238225, JP-A-2000-40217 and JP-A-2000-40218. In these publications, the skew of output from the MR head is prevented with controlling the magnetic flux of the magnetic recording medium (a product of a residual magnetic flux density and a thickness), or the thermal asperity of the MR head is suppressed with reducing dents on the surface of the magnetic layer to a specific value or less.
The conventional magnetic head uses a chip itself, which is a laminate of a magnetic induction type head for recording and a magnetic induction type head for reading. On the other hand, the MR head 20 is combined with the magnetic induction type head 21 for recording and embedded in the slider 22, as schematically shown in FIGS. 2 and 3. In FIGS. 2 and 3, numeral 20a stands for the MR element, 21a and 21b for magnetic elements composing the recording head 21, 21c for a reading gap, and 23 for a shielding member. The MR head is embedded with receding from the slider surface 22a by about 25 nm.
That is, the conventional head consists of a very small chip, and thus the magnetic tape runs over the head as if a knife edge bites the tape, while, in the case of the MR head, the magnetic tape 30 runs with contacting to the slider 22, since the MR head 20 is embedded in the large slider 22 with receding from the slider surface (as shown in FIGS. 2 and 3). The magnetic tape 30 and the MR head 20 are in contact with each other as if the magnetic tape 30 expands towards the MR head 20. Since the contacting state of the MR head with the magnetic tape is very different from that of the conventional magnetic head, properties required for the magnetic tape change completely in connection with the decrease of the spacing loss.
Furthermore, since the MR element 20a of the MR head 20 comprises a very thin film, it is easily worn out. As shown in FIGS. 2 and 3, usually a pair of the MR head 20 and the recording head 21 are used so that the magnetic tape can be recorded and read when it runs either in a forward direction or in a backward direction. Furthermore, a plurality of pairs of the MR head and the recording head are provided as shown in FIG. 2 so that a plurality of tracks can be recorded and read at the same time.
In addition, since the MR head has a very narrow track width, a servo-signal is provided for the tracking servo of the MR head.
The track servo system includes a magnetic servo system and an optical servo system. The former system magnetically records a servo-band in the magnetic layer and reads the servo-band to carry out servotracking, while the latter system forms a servo-band comprising depression arrays with laser irradiation, etc. and optically reads the servo-band to carry out the servotracking. Besides the above systems, the magnetic servo system includes a system in which magnetism is imparted to the backcoat layer, and a magnetic serve-signal is recorded in the backcoat layer, and the optical servo system includes a system in which an optical servo signal is recorded in a backcoat layer with a material that absorbs light.
To keep up with the increase of the traveling speed of the magnetic tape and the relative speed between the tape and the head, it is necessary to travel the tape at a high speed while tracing the servo-signal. However, if a coefficient of friction of the magnetic layer or the backcoat layer against a slider material (for example, alumina/titania/carbide, etc.) or a material of guide rollers is insufficiently optimized, the magnetic tape meanders so that the tracking may deviate (off-track), PW50 may increase or the output may decrease. Therefore, the error rate may increase.
One object of the present invention is to provide a magnetic recording medium which can decrease the spacing loss of the medium to be used with the MR head, and reduce the error rate through the suppression of the off-track caused by the meandering of the medium.
Another object of the present invention is to provide a magnetic recording medium which can reduce the wearing of the MR head to be used in combination with the magnetic recording medium.
To achieve the above objects, extensive research has been done. As a result, it has been found that, when a difference of the height of the highest projection (P1) and the averaged height of the projections (P0) (P1xe2x88x92P0) is smaller than a specific value, and the difference of the height of the highest projection (P1) and the height of the 20th highest projection P20 (P1xe2x88x92P20) is also smaller than a specific value, a spacing loss is decreased, and further that, when a ratio of xcexcmSL to xcexcmSUS [(xcexcmSL)/(xcexcmSUS)] and a ratio of xcexcmSL to xcexcBSUS [(xcexcmSL)/(xcexcBSUS)], wherein xcexcmSL is a coefficient of kinetic friction (hereinafter referred to as xe2x80x9ccoefficient of frictionxe2x80x9d) between a magnetic layer and a slider material (e.g. alumina/titania/carbide), xcexcmSUS is a coefficient of friction between the magnetic layer and a stainless steel (SUS304) (hereinafter referred to as xe2x80x9cSUSxe2x80x9d), and xcexcBSUS is a coefficient of friction between the backcoat layer and SUS, are controlled to specific values, the off-track caused by the meandering of the magnetic tape is suppressed and thus an error rate is improved. The effect to improve the error rate through the decrease of the off-track is particularly remarkable when the track width is 15 xcexcm or less.
The present invention has been completed based on the above findings.
Accordingly, the above objects are achieved by a magnetic recording medium comprising a non-magnetic support, at least one primer layer formed on one surface of the non-magnetic support, a magnetic layer formed on the primer layer, and a backcoat layer formed on the other surface of the non-magnetic support, wherein the magnetic layer has a thickness of 0.30 xcexcm or less and a centerline average surface roughness Ra of 3.2 nm or less, and (P1xe2x88x92P0) is 30 nm or less and (P1xe2x88x92P20) is 5 nm or less in which P0 is an averaged height of projections of the magnetic layer, and P1, P2, - - - and P20 are heights of the highest, the second highest, - - - and the 20th highest projections of the magnetic layer, respectively.
The magnetic recording medium of the present invention is preferably used in applications where magnetically recorded signals are read with a reading head comprising a magnetoresistatance element.
In one preferred embodiment of the present invention, a ratio of xcexcmSL to xcexcmSUS [(xcexcmSL)/(xcexcmSUS)] is from 0.7 to 1.3 and a ratio of xcexcmSL to xcexcBSUS [(xcexcmSL)/(xcexcBSUS)] is from 0.8 to 1.5, wherein xcexcmSL is a coefficient of friction between the magnetic layer and a slider material, xcexcmSUS is a coefficient of friction between the magnetic layer and SUS, and xcexcBSUS is a coefficient of friction between the backcoat layer and SUS.
In another preferred embodiment of the present invention, the magnetic layer has a coercive force of 120 to 320 kA/m, and a product of a residual magnetic flux density in the machine direction of the magnetic layer and the thickness of the magnetic layer is from 0.0018 xcexcTm to 0.06 xcexcTm.
In a further preferred embodiment of the present invention, the backcoat layer contains small particle size carbon black having a particle size of 5 to 100 nm and large particle size carbon black having a particle size of 200 to 400 nm in a total amount of 60 to 98% by weight based on the weight of inorganic powders in the backcoat layer and iron oxide particles having a particle size of 0.1 to 0.6 xcexcm in an amount of 2 to 40% by weight based on the weight of inorganic powders in the backcoat layer, and has a centerline average surface roughness Ra of 2 to 15 nm.
In a yet further preferred embodiment of the present invention, the non-magnetic support has a thickness of 7.0 xcexcm or less, a Young""s modulus in the machine direction of at least 6.08 GPa (at least 600 kg/mm2), and a ratio of a Young""s modulus MD in the machine direction to a Young""s modulus TD in the transverse direction (MD/TD) is from 0.6 to 1.80.
The coefficient of friction used herein means a coefficient of kinetic friction, and the measurement method thereof will be explained in detail in below described Examples.